The present disclosure relates generally to a system for, and a method of, estimating bearings of radio frequency (RF) identification (RFID) tags associated with items located in a controlled area, especially when the tags return RFID receive signals whose power is below a predetermined threshold due to such real-world conditions as multi-path reflections, destructive signal interference, ambient temperature variations, etc.
Radio frequency (RF) identification (RFID) technology is becoming increasingly important for logistics concerns, material handling and inventory management in retail stores, warehouses, distribution centers, buildings, and like controlled areas. An RFID system typically includes an RFID reader, also known as an RFID interrogator, and preferably a plurality of such readers distributed about the controlled area. Each RFID reader interrogates at least one RFID tag, and preferably many more RFID tags, in its coverage range. Each RFID tag is usually attached to, or associated with, an individual item, or to a package for the item, or to a pallet or container for multiple items. Each RFID tag typically includes an antenna, a power management section, a radio section, and frequently a logic section containing a control microprocessor, a memory, or both. Each RFID reader transmits an RF interrogating signal, and each RFID tag, which senses the interrogating RF signal, responds by transmitting a return RFID receive signal. The RFID tag either generates the return RFID receive signal originally, or reflects back a portion of the interrogating RF signal in a process known as backscatter. The return RFID receive signal may further encode data stored internally in the tag. The return RFID receive signal is demodulated and decoded into data by each reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data, also known as a payload, can denote a serial number, a price, a date, a destination, other attribute(s), or any combination of attributes, and so on.
The RFID system is often used in an inventory monitoring application. For example, in order to take inventory of RFID-tagged items in a retail store, it is known to position at least one RFID reader overhead in a controlled area, and then, to allow each reader to automatically read whatever tagged items are in the coverage range of each reader. For superior RF coverage, it is known to provide each reader with at least one overhead array of antenna elements that are arranged about a central vertical axis, also known as a plumb line, and that transmit the RF interrogating signal as a primary transmit beam that is electronically steered both in azimuth and in elevation, and that receive a return primary receive signal via a primary receive beam from the tags.
As satisfactory as such known RFID systems utilizing antenna arrays have been in monitoring inventory, they can also be used for locationing applications, i.e., for estimating and determining the bearing, i.e., the angular direction both in azimuth and elevation, of any particular tag, relative to a particular reader. However, there is a practical limit on the number of antenna elements that can be used in each array. This antenna element limit causes each primary transmit beam and each corresponding primary receive beam to have a relatively broad beam width. The primary transmit beam is typically steered until the reader reads the tag with the highest power or peak receive signal strength indicator (RSSI) of the return primary receive signal at a primary steering angle. However, estimating the bearing, i.e., the angular direction both in azimuth and elevation, of any particular tag based on the peak RSSI of the return primary receive signal is imprecise due to the aforementioned relatively broad beam width. Bearing errors on the order of 5 to 10 degrees have been reported and are not readily tolerable in locationing applications.
To improve the accuracy of estimating the location of a particular tag and obtain a true bearing, it is known to generate multiple secondary receive beams pointing in different directions, and to respectively capture return secondary receive signals. The primary and the secondary receive beams are jointly moved together, as a unit, in a search pattern or path in the controlled area. The controlled area is divided into multiple sectors or zones, in which the joint unit movement of the primary and the secondary receive beams is performed at multiple primary steering angles in each sector. The secondary receive signals are processed to generate azimuth and elevation error signals as azimuth and elevation corrections to the primary steering angle of the primary receive beam, thereby reducing the bearing error.
Yet, as advantageous as the known RFID system has been in accurately locating the true bearings of tags generally located in the controlled area, experience has shown that there are times when real-world conditions may sometimes interfere with the generation and processing of the return primary and secondary receive signals. For example, the controlled area may contain shelving, fixtures, equipment, vehicles, and the like, not to mention the floor, the ceiling and the room walls, each or all of which can reflect and scatter the primary and/or secondary receive beams incident thereon, thereby compromising the generation and processing of their corresponding receive signals such that one or more of such receive signals have no or low power, i.e., their RSSI is below a minimum acceptable threshold, due to multi-path reflections, destructive interference among signals, ambient temperature variations, etc. As a result, the known RFID system cannot always accurately estimate the true bearing of a tag for such a real-world environment.
Accordingly, there is a need to estimate the bearings of RFID tags located anywhere in a controlled area, especially in a real-world environment where such receive signals have no or low power.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and locations of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.